1. Field of the Invention
The present invention relates to a system for receiving broadcast digital data comprising a master digital terminal, and at least one slave digital terminal connected to the master terminal.
2. Description of the Related Art
The market for digital television decoders is currently reaching a turning point. Most subscribers, in the European Countries in particular, are equipped with a single digital terminal (or <<decoder>>) per household, where as they often possess at least two television sets. There therefore exists a demand for multiple equipment in terms of decoders for one and the same household.
It will be noted that subsequently the terms <<decoder>> or <<digital terminal>> designate one and the same type of device making it possible to receive and decode (and possibly to descramble) digital signals broadcast by an operator (in particular a digital television operator). In the subsequent description use will also be made of the terms <<scramble>>/<<descramble>> or <<encipher>>/<<decipher>> to signify that an encryption/decryption algorithm is applied to data using a key.
Certain operators of pay digital television wish to offer their subscribers the possibility of equipping themselves with several digital terminals so as to benefit from their services on each of the television sets installed in their accommodation, without however making them pay the price of a full tariff subscription for the additional terminals, which would be prohibited, but rather a reduced tariff (or even a zero tariff). However, the operator has to ensure that the terminals and <<associated>> subscriptions actually remain within the same household, since in the converse case, their income is at risk of being considerably affected thereby.
A known solution consists in using the <<return path>> of the digital terminals by requesting the subscriber to link all the terminals of his home to one and the same telephone line. The operator then periodically monitors the connection of the terminals to this telephone line by remotely instructing telephone calls from the terminals to a server of the operator. However, this solution is not satisfactory since it entails the permanent connection of the digital terminals of the subscriber to a telephone line.
Another solution described in French Patent Application No. 02 09362 filed on 24 Jul. 2002 by the same applicant as the present application, THOMSON Licensing S. A., consists in guaranteeing that a physical communication link always exists between a secondary terminal (or <<slave>> terminal) and a main terminal (or <<master>> terminal) with which it is paired. The slave terminal or terminals (for which the subscriber benefits from a preferential tariff) cannot operate, that is to say provide data in clear to the television set to which they are connected, unless it is verified that the <<master>> terminal with which they are paired is present in proximity.
Several strategies for communication between these decoders are conceivable but some of them may exhibit risks of <<piracy>> or of <<circumvention>>.